This invention relates to the application of a diffusion aluminide coating on a surface, and in particular, to the application of such a coating from an aluminum-containing paint.
In an aircraft gas turbine (jet) engine, air is drawn into the front of the engine, compressed by a shaft-mounted compressor, and mixed with fuel. The mixture is burned, and the hot exhaust gases are passed through a turbine mounted on the same shaft. The flow of combustion gas turns the turbine by impingement against an airfoil section of the turbine blades and vanes, which turns the shaft and provides power to the compressor and fan. In a more complex version of the gas turbine engine, the compressor and a high pressure turbine are mounted on one shaft, and the fan and low pressure turbine are mounted on a separate shaft. The hot exhaust gases flow from the back of the engine, driving it and the aircraft forward.
The hotter the combustion and exhaust gases, the more efficient is the operation of the jet engine. There is thus an incentive to raise the combustion and exhaust gas temperatures. The maximum temperature of the combustion gases is normally limited by the materials used to fabricate the turbine vanes and turbine blades of the turbine, upon which the hot combustion gases impinge. In current engines, the turbine vanes and blades are made of nickel-based superalloys, and can operate at temperatures of up to about 1900-2150xc2x0 F.
Many approaches have been used to increase the operating temperature limits of turbine blades, turbine vanes, and other hot-section components to their current levels. In one such approach, the article is protected by a diffusion aluminide protective coating with an optional ceramic thermal barrier coating overlying the protective coating.
During service, the portions of the hot-section components that are exposed to the most severe conditions of temperature and environment are sometimes damaged so that some of the underlying material is lost, thereby changing the dimensions of the hot-section component and adversely affecting its functionality. When such damage is detected, the damaged article is removed from service. If the damage is not too extreme, the damaged article may be repaired and later returned to service.
When the article is repaired, the protective coating and the thermal barrier coating may be removed, and the damaged region is built up by welding. A new protective coating and new thermal barrier coating may be deposited to complete the repair.
The removal of the entire protective coating and thermal barrier coating, and the deposition of new coatings, is an expensive and time-consuming process. For some types of repairs, it would be desirable to leave these coatings in place in those areas which are not being repaired, so that only the repaired area would be recoated. However, this partial recoating is not possible with the existing approach for applying the diffusion aluminide protective coating.
There is accordingly a need for an improved approach to applying a protective coating to hot-section components of gas turbine engines and comparable articles, which permits partial removal of the protective coating and partial recoating. The present invention fulfills this need, and further provides related advantages.
The present invention provides a technique for applying a diffusion aluminide coating to a surface of an article such as a nickel-base superalloy. The coating is performed by painting techniques from a paint composition that is distinct from a slurry that contains a halide activator. Upon heating, the coating proceeds by a condensed-phase (solid or liquid) reaction. There is substantially no gaseous phase beyond ordinary vapor pressure that can coat those portions of the article which are desirably not coated. Vapor-phase masking of these portions that are not to be coated is not required during heating, although paint masking may be used during application of the coating precursor paint in the same manner as conventional painting.
A method for coating an article includes providing an article to be coated such as a gas turbine component made of a nickel-base superalloy. A coating precursor paint comprising aluminum-containing pigment particles, a temporary thin-film-forming binder comprising an organic resin such as an acrylic, and a solvent for the temporary binder is prepared. The coating precursor paint is applied to a surface of the article, and thereafter the coating precursor paint is heated to a temperature of from about 1200xc2x0 F. to about 2100xc2x0 F. in a non-oxidizing environment. The heating step may be performed in vacuum, a non-oxidizing atmosphere such as an inert gas, or in a low partial pressure of oxygen gas in some cases.
The aluminum-containing pigment particles may be substantially pure aluminum. In another embodiment, the aluminum-containing pigment particles may comprise aluminum and at least one other alloying element selected from group consisting of platinum, hafnium, zirconium, yttrium, lanthanum, cerium, chromium, palladium, silicon, nickel, cobalt, and titanium, and mixtures thereof.
The temporary organic binder is present to aid in holding the pigment particles together during processing and to the surface of the article during application and prior to completion of the heating step. The temporary organic binder is depolymerized during heating and leaves little if any residue in the final diffusion aluminide coating. The temporary binder is an organic material, preferably an acrylic, and most preferably a methacrylate such as butyl methacrylate resin, ethyl methacrylate resin, methyl methacrylate resin, or methacrylate co-polymer resin. Other less-preferred temporary organic binders include alkyd resins, shellac, rosin, rosin derivatives, ester gum, vinyls, styrenics, polyesters, epoxies, polyurethanes, and cellulose derivatives, and mixtures thereof.
Other layers may optionally be applied in conjunction with the diffusion aluminide coating. For example, a first coating layer comprising platinum, palladium, or chromium may be applied to the surface of the article prior to coating with the diffusion aluminide. A ceramic thermal barrier layer may be applied overlying the diffusion aluminide coating.
The coating precursor paint may be applied by any operable method, with examples being dipping, brushing, and spraying. The amount of solvent is selected consistent with the application technique. Spray painting requires more fluidity and thence more solvent than does brushing, for example.
There is no activator such as a halide activator present in the coating precursor paint. The activator is used in slurries to effect coating by a vapor phase mass transport from the source particles to the surface being coated. While the activator approach is operable and widely used, it has the disadvantage that the aluminum-containing vapor is difficult to contain so that it does not coat portions of the article surface that are desirably left uncoated. Vapor-phase maskants to prevent penetration and coating by the aluminum-containing vapor are known, but these vapor-phase maskants are difficult to use and not always fully effective. In the present case, the aluminum is transported to the surface of the article in a condensed phase, typically the liquid phase, and there is very little aluminum vapor present except for that associated with the normal vapor pressure. Thus, in some cases the coating precursor paint of the present invention may be applied to only a portion of the surface of the article. The step of heating the coating precursor paint may be performed without a vapor-phase mask overlying the portion of the surface of the article to which no coating precursor paint is to be applied and is not to be coated.
The present invention thus provides a technique for applying a diffusion aluminide coating to a surface. The source of the diffusion aluminide coating is a non-activated paint that is mixed and applied by conventional painting techniques. The diffusion aluminide coating is confined to the areas that are initially coated with the coating precursor paint, without the need for a vapor-phase mask to be present during the heating that transforms the applied precursor paint to a diffusion aluminide coating.
The present approach is particularly advantageously utilized in repair operations. The halide activator used in other techniques for applying a diffusion aluminide coating may chemically attack the portion of the ceramic thermal barrier coating that is not removed in the repair process. The present approach has no halide activator, and therefore the paint may be applied in areas adjacent to those where the pre-existing diffusion aluminide coating and ceramic thermal barrier coating have not been removed without concern that there will be a halide activator present to attack the ceramic thermal barrier coating. As a result, the present approach allows a partial removal of the pre-existing coating only in the areas where there is to be a repair, and then recoating of those areas after the repair, without damaging the pre-existing coating.